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  • 51. Larsson, Andreas
    Adiabatic internal modes: a new way to gain insights from molecular vibrational spectroscopy2000Konferensbidrag (Refereegranskat)
  • 52. Larsson, Andreas
    DFT Modelling of Single Molecule Spectoscopy of Fullerenes and Metal Phthalocyanines2010Konferensbidrag (Refereegranskat)
  • 53. Larsson, Andreas
    Modelling Nanostructured Carbon Materials for Future ICT Applications2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    Carbon nanotubes (CNTs) have, due to their remarkable mechanical, electronic and thermal properties, many suggested uses, and have even been demonstrated as interconnects and nano-transistors in laboratory built devices [1-4]. This is an important effort since metallic CNTs have lower resistivity than Cu, and semiconducting CNTs have higher mobility than Si, which are traits that could help push the boundaries of Moore s law even further. The reason CNTs are not yet incorporated into electronics is due to growth control and placement issues. With present day state-of-the-art techniques it is not possible to grow CNTs with only one property (i.e. either all metallic or all semiconducting), which presents the first and principal hurdle for the utilisation of CNTs in semiconductor industry. It is, however, possible to grow CNTs of a certain type (multi-walled, double-walled, or single walled), within a rather narrow diameter distribution. It is also well understood how the orientation of the honey-comb structure relative to the CNT axis determines the property of the CNT itself. The problem lies in realizing growth of CNTs with control over this internal graphene structuring. We have performed first-principles calculations of how single-walled carbon nanotubes (SWNTs) bond with different metal nanoparticles explaining why the traditional catalysts (Fe, Co, Ni) are more successful than other metals (Cu, Pd, Au) [5], and how this realization relates to new nanocomposite catalyst particles (Cu/Mo) [6]. We will present our contribution to understanding the mechanism of CNT growth, since it is only through better knowledge that property-controlled growth of CNTs can be achieved

  • 54. Larsson, Andreas
    et al.
    Cremer, Dieter
    Department of Theoretical Chemistry, Göteborg University.
    Theoretical verification and extension of the McKean relationship between bond lengths and stretching frequencies1999Ingår i: Journal of Molecular Structure, ISSN 0022-2860, E-ISSN 1872-8014, Vol. 485-486, s. 385-407Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Vibrational spectra contain explicit information on the electronic structure and the bonding situation of a molecule, which can be obtained by transforming the vibrational normal modes of a molecule into appropriate internal coordinate modes, which are localized in a fragment of the molecule and which are associated to that internal coordinate that describes the molecular fragment in question. It is shown that the adiabatic internal modes derived recently (Int. J. Quant. Chem., 67 (1998) 1) are the theoretical counterparts of McKean's isolated CH stretching modes (Chem. Soc. Rev., 7 (1978) 399). Adiabatic CH stretching frequencies obtained from experimental vibrational spectra can be used to determine CH bond lengths with high accuracy. Contrary to the concept of isolated stretching frequencies a generalization to any bond of a molecule is possible as is demonstrated for the CC stretching frequencies. While normal mode frequencies do not provide a basis to determine CC bond lengths and CC bond strengths, this is possible with the help of the adiabatic CC stretching frequencies. Measured vibrational spectra are used to describe different types of CC bonds in a quantitative way. For CH bonds, it is also shown that adiabatic stretching frequency leads to the definition of an ideal dissociation energy, which contrary to the experimentally determined dissociation energy is a direct measure of the bond strength. The difference between measured and ideal dissociation energies gives information on stabilization or destabilization of the radicals formed in a dissociation process

  • 55. Larsson, Andreas
    et al.
    Delaney, Paul
    Tyndall National Institute, University College Cork.
    Electronic structure of the nitrogen-vacancy center in diamond from first-principles theory2008Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, nr 16Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The nitrogen-vacancy (NV) center is a paramagnetic defect in diamond with applications as a qubit. Here, we investigate its electronic structure by using ab initio density functional theory for five different NV center models of two different cluster sizes. We describe the symmetry and energetics of the low-lying states and compare the optical frequencies obtained to experimental results. We compute the major transition of the negatively charged NV centers to within 25-100 meV accuracy and find that it is energetically favorable for substitutional nitrogens to donate an electron to NV0. The excited state of the major transition and the NV0 state with a neutral donor nitrogen are found to be close in energy

  • 56. Larsson, Andreas
    et al.
    Delaney, Paul
    Queen's University Belfast.
    First-Principles Calculations of the Electronic Structure of the Nitrogen-Vacancy Center in Diamond2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    The Nitrogen-Vacancy or NV-center is a color center in diamonds containing atomic nitrogen [1]; in which the nitrogen and the vacancy are situated on neighboring lattice positions. The defect has C3v symmetry; with the nitrogen and the vacancy lying on the C3-axis. It has been well characterized experimentally: usually it is singly negatively charged; has a paramagnetic ground state (S=1) with spatial symmetry A2; and it has a strong dipole allowed 3A2 to 3E transition at 1.945 eV [2]. Recently the NV-center has been used as a qubit; and quantum NOT and controlled rotation (CROT) gates have been demonstrated [3] as well as optical read-out of the electronic spin state [4]. The NV-center has also been used as a single photon source [5;6]. Creating NV-centers in nanodiamonds could open up the possibility of placing NV-centers in an array as qubits in a quantum computer. In this work we present transition energies calculated from first-principles density functional theory (DFT) of the NV-center incorporated into two hydrogen-terminated nanodiamonds with approximate diameters of 1.2 nm and 1.5 nm [7]. We describe the symmetry and energetics of the low-lying states and compare the optical frequencies obtained to experimental results. We compute the major transition of the negatively charged NV-center to within 25 100 meV accuracy and find that it is energetically favourable for substitutional nitrogens to donate an electron to NV0. The excited state of the major transition and the NV0 state with a neutral donor nitrogen are found to be close in energy.

  • 57. Larsson, Andreas
    et al.
    Delaney, Paul
    Tyndall National Institute, University College Cork.
    Nanodiamonds2005Konferensbidrag (Refereegranskat)
  • 58. Larsson, Andreas
    et al.
    Delaney, Paul
    Queen's University Belfast.
    Theoretical investigation of the hardness of nanodiamonds and the energetics of the NV-centre2006Konferensbidrag (Refereegranskat)
  • 59. Larsson, Andreas
    et al.
    Elliott, Simon D.
    Tyndall National Institute, University College Cork.
    Greer, James C.
    Tyndall National Institute, University College Cork.
    Repp, Jascha
    IBM Research, Zurich Research Laboratory.
    Meyer, Gerd
    IBM Research, Zurich Research Laboratory.
    Allenspach, Rolf
    IBM Research, Zurich Research Laboratory.
    Orientation of individual C60 molecules adsorbed on Cu(111): Low-temperature scanning tunneling microscopy and density functional calculations2008Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, nr 11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Density functional theory (DFT) and low-temperature scanning tunneling microscopy (STM) have been combined to examine the bonding of individual C60 molecules on Cu(111). Energy-resolved differential-conductance maps have been measured for individual C60 molecules adsorbed on a Cu(111) surface by means of low-temperature STM, which are compared to and complemented by theoretically computed spectral images. It has been found that C60 chemisorbs with a six-membered ring parallel to the surface at two different Cu(111) binding sites that constitute two exclusive hexagonal sublattices. On each sublattice, C60 is bonded in one particular rotational conformer, i.e., C60 molecules bind to the Cu(111) surface in two different azimuthal orientations differing by 60°depending on which sublattice the binding site belongs to. The binding conformation of C60 and its orientation with regard to the copper surface can be deduced by this joint experimental-theoretical approach. Six possible pairs of C60 configurations on three different Cu surface binding sites have been identified that fulfil the requirements of the two sublattices and are consistent with all experimental and theoretical data. Theory proposes that two of these configuration pairs are most likely. We have found that DFT does not get the binding energy between rotational conformers in the correct order. We also report two different C60 monolayers on Cu(111): one with alternating orientations of neighboring molecules at low temperature and the other with (4×4) structure after annealing above room temperature.

  • 60. Larsson, Andreas
    et al.
    Fagas, G.
    Greer, James C.
    National Institute, Lee Makings, Prospect Row, Cork.
    Martinez, C.
    Patterson, J.
    Nikonov, D.
    Park, S.
    Haverty, M.G.
    Shankar, S.
    Multi-Scale Simulation for Nanowires and Carbon Nanotubes2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    With the advent of nanotechnology; semiconductor processing is driving developments in computational chemis-try and computational material science. Describing materials from an atomic perspective allows for simulations that describe the formation of and flow of electrons in nanoscale structures. However; to achieve design for mi-croelectronics technologies requires atomic scale simulations that span varying length and time scales. In this presentation; developments toward a strategy for simulating nanowire and carbon nanotubes; their electronic structure; and electron and phonon transport within a single simulation hierarchy is described.

  • 61. Larsson, Andreas
    et al.
    Greer, James C.
    NMRC, University College, Lee Maltings, Cork.
    A first principle s investigation of endohedral group V eelments X@C60 and their applications to nanotehnology and quantum computing2001Konferensbidrag (Refereegranskat)
  • 62. Larsson, Andreas
    et al.
    Greer, James C.
    NMRC, University College, Lee Maltings, Cork.
    Nitrogen and Phosphorus Trapped in Buckminsterfullerene2002Artikel i tidskrift (Refereegranskat)
  • 63. Larsson, Andreas
    et al.
    Greer, James C.
    NMRC, University College, Lee Maltings, Cork.
    Structural and electronic properties of endohedral phosphorus fullerene P@C60: An off-centre displacement of P inside the cage2002Ingår i: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 100, nr 21, s. 3475-3477Artikel i tidskrift (Refereegranskat)
  • 64. Larsson, Andreas
    et al.
    Greer, James C.
    NMRC, University College, Lee Maltings, Cork.
    The use of fullerenes and endohedrally doped fullerenes as building blocks in nanothenoogy2002Konferensbidrag (Refereegranskat)
  • 65. Larsson, Andreas
    et al.
    Greer, James C.
    National Microelectronics Research Centre (NMRC), University College, Lee Maltings, Prospect Row, Cork.
    Harneit, Wolfgang
    Hahn-Meitner Institute, Glienicker Str. 100, D-14109 Berlin.
    Weidinger, Alois
    Hahn-Meitner Institute, Glienicker Str. 100, D-14109 Berlin.
    Phosphorous trapped within buckminsterfullerene2002Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 116, nr 18, s. 7849-7854Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The chemical, spin, and thermodynamic properties of endohedral phosphorous were studied. A model for the interaction between the trapped group V atom and the fullerene cage was also discussed. It was observed that the repulsion between the group V atom's valence shell and the fullerene cage result in an exothermic binding of the atom within the cage.

  • 66. Larsson, Andreas
    et al.
    Kolodziejczyk, J.
    Baran, J.D.
    Larsson, Peter O.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Ahuja, Rajeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Bolton, Kim
    Physics Department, Göteborg University.
    Ding, Feng
    Physics Department, Göteborg University.
    Rosen, Arne E.
    Physics Department, Göteborg University.
    Modelling of Carbon Nanotube Catalytic Growth2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    Carbon nanotubes (CNTs) have; due to their remarkable mechanical; electronic and thermal properties; many suggested uses; and have even been demonstrated as interconnects and nano-transistors in laboratory built devices [1-4]. The reason CNTs are not yet incorporated into electronics is due to growth control and placement issues. With present day state-of-the-art techniques it is not possible to grow CNTs with only one property (i.e. either all metallic or all semiconducting); which presents the first and principal hurdle for the utilisation of CNTs in semiconductor industry. It is; however; possible to grow CNTs of a certain type (multi-walled; double-walled; or single walled); within a rather narrow diameter distribution. It is also well understood how the orientation of the honey-comb structure relative to the CNT axis determines the property of the CNT itself. The problem lies in realizing growth of CNTs with control over this internal graphene structuring. We have performed first-principles calculations of how single-walled carbon nanotubes (SWNTs) bond with different metal nanoparticles explaining why the traditional catalysts (Fe; Co; Ni) are more successful than other metals (Cu; Pd; Au) [5]; and how this realization relates to new nanocomposite catalyst particles (Cu/Mo) [6]. We will present our contribution to understanding the mechanism of catalytic CNT growth; since it is only through better knowledge that property-controlled growth of CNTs can be achieved

  • 67. Larsson, Andreas
    et al.
    Nolan, Michael
    NMRC, University College, Lee Maltings, Cork.
    Greer, James C.
    NMRC, University College, Lee Maltings, Cork.
    Interactions between thiol molecular linkers and the Au13 nanoparticle2002Ingår i: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, s. 5931-5937Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The structure and binding of thiol molecular linkers to gold surfaces and nanoparticles is central to the understanding of the electronic properties of self-assembled monolayers and, of relevance to recent studies, to nanoscale assemblages consisting of molecular wires and metal nanoparticles. The study of mono-molecular electron transport generally requires consideration of bonding with irregular metallic contacts or poorly defined surfaces such as break junctions, electromigration generated gaps, and scanning probe microscopy tips. These structures can locally bear a closer resemblance to atomic clusters, as compared to neat metallic surfaces. It has also emerged that the prediction and understanding of the electronic transport properties for molecular wires and nanoscale assemblies requires detailed knowledge of thiolate-gold cluster interactions. Recent debate has focused on the nature of the thiolate bonding to surfaces, and the effect of disordering and distortion in gold cluster structures on thiolate bonding. We apply density functional theory methods to study the interactions of two thiols-methanethiol and benzenethiol-with Au13, a gold "magic" number cluster. Our study emphasizes the effects of thiolate bonding on the electronic structure of the linker molecule and gold cluster. We find significant local distortion of the gold cluster upon bonding to a thiol group, resulting in modifications to the electronic structure of the complex. Consideration of a finite gold cluster avoids many of the issues related to thiolate bonding on gold surfaces, and allows us to assess the impact of bonding to gold nanoparticles in terms of electronic structure. We discuss our findings in relation to electron transport properties in self-assembled systems.

  • 68. Larsson, Andreas
    et al.
    Tong, Longyu
    NMRC, University College, Lee Maltings, Cork.
    Cheng, Tongwei
    NMRC, University College, Lee Maltings, Cork.
    Nolan, Michael
    NMRC, University College, Lee Maltings, Cork.
    Greer, James C.
    NMRC, University College, Lee Maltings, Cork.
    Basis set study for the calculation of electronic excitations using Monte Carlo configuration interaction2001Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 114, nr 1, s. 15-22Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A recipe for the calculation of excited states was presented, which constituted a significant step forward in making accurate excited state calculations. An acceptable energy difference between experimental and calculated energies to be less than 100 meV were also defined. The importance of a balanced description of the excited states and the ground state was emphasized and the resulting electronic transitions were compared with experimental values.

  • 69. Larsson, Peter
    et al.
    Ding, Feng
    Physics Department, Göteborg University.
    Larsson, Andreas
    Rosén, Arne E.
    Physics Department, Göteborg University.
    Aruja, Rejeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Bolton, Kim
    Physics Department, Göteborg University.
    Carbon nanotube-metal cluster bond strenghts: implications for catalytic growth2006Konferensbidrag (Refereegranskat)
  • 70.
    Larsson, Peter
    et al.
    Department of Physics, University of Uppsala.
    Araújo, Carlos Moysés
    Department of Physics, University of Uppsala.
    Larsson, Andreas
    Jena, Puru
    Department of Physics, Virginia Commonwealth University, Richmond.
    Ahuja, Rajeev B.
    Department of Physics, University of Uppsala.
    Theoretical investigation of catalysed MgH22006Konferensbidrag (Refereegranskat)
    Abstract [en]

    MgH2 has attracted much attention for being a good hydrogen storage material due to its light weight, low manufacturing cost and high storage capacity (7.6 wt%). But its slow absorption/desorption kinetics and high dissociation temperature (nearly 300 C) limit its practical applications for hydrogen storage. To overcome this, much effort has been paid mainly by making nanocrystalline Mg and/or by adding alloying elements. In this work, we provide a theoretical investigation of the electronic and structural properties of pure and M-doped MgH2 (with M=Sc, Ti, V, Fe, Ni, Al). We have made calculations for both the crystalline state and 1.0 nm particles. The self-consistent total energy calculations are performed within density functional theory using the VASP package for crystals and TURBOMOLE package for clusters. One aim of this study is to see if the alloying elements can weaken the Mg-H bonds, resulting in improved thermodynamics and faster kinetics. Another one is to understand the differences in the thermodynamics of clusters and crystals

  • 71.
    Larsson, Peter
    et al.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Ding, Feng
    Physics Department, Göteborg University.
    Larsson, Andreas
    Ahuja, Rejeev B.
    Condensed Matter Theory Group, Department of Physics, Uppsala Universitet.
    Rosén, Arne E.
    Physics Department, Göteborg University.
    Bolton, Kim
    Physics Department, Göteborg University.
    Bond strength between single-walled carbon nanotubes (SWNTs) and metal clusters: implication for catalytic growth2006Konferensbidrag (Refereegranskat)
  • 72.
    Larsson, Peter O.
    et al.
    Department of Physics, University of Uppsala.
    Araúj, Carlos Moysés
    Department of Physics, University of Uppsala.
    Larsson, Andreas
    Ahuja, Rajeev B.
    Department of Physics, University of Uppsala.
    An ab initio study of the crystalline and nanostructured pure and m-dpoed MgH22005Konferensbidrag (Refereegranskat)
  • 73.
    Larsson, Peter O.
    et al.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Araújo, Carlos Moysés
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Larsson, Andreas
    Jena, Puru
    Department of Physics, Virginia Commonwealth University, Richmond.
    Ahuja, Rajeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    A detailed look at the catalytic action of transition metal atom dopants in MgH2 nanoclusters2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    Magnesium hydride (MgH2 ) is a candidate for solid-state hydrogen storage applications. In this work; hydrogen desorption from transition metal doped MgH2 clusters of approximately 1 nm was investigated by density functional theory (PBE functional and Gaussian basis sets). It was found that transition metals were more stable at the surface than in the center of the cluster and that desorption energies of hydrogen atoms bound to such surface doped transition metals were significantly lowered. It was furthermore observed how transition metals attract hydrogens to keep at least four hydrogen atoms coordinated even when the total hydrogen content of the cluster decreases. This effect is associated with migration of the transition metal atoms from the surface sites to the interior sites during the dehydrogenation process; releasing more hydrogen as they diffuse. This diffusion mechanism may account for the fact that a small amount of catalysts is sufficient to improve the kinetics of MgH2 .

  • 74.
    Larsson, Peter O.
    et al.
    Department of Physics, University of Uppsala.
    Araújo, Carlos Moysés
    Department of Physics, University of Uppsala.
    Larsson, Andreas
    Jena, Puru
    Department of Physics, Virginia Commonwealth University, Richmond.
    Ahuja, Rajeev B.
    Department of Physics, University of Uppsala.
    Activities and whereabouts of transition metal atom catalysts in MgH2 nanoclusters2007Konferensbidrag (Refereegranskat)
  • 75.
    Larsson, Peter O.
    et al.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Araújo, Carlos Moysés
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Larsson, Andreas
    Jena, Puru
    Department of Physics, Virginia Commonwealth University, Richmond.
    Ahuja, Rajeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Role of catalysts in dehydrogenation of MgH2 nanoclusters2008Ingår i: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 105, nr 24Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A fundamental understanding of the role of catalysts in dehydrogenation of MgH2 nanoclusters is provided by carrying out first-principles calculations based on density functional theory. It is shown that the transition metal atoms Ti, V, Fe, and Ni not only lower desorption energies significantly but also continue to attract at least four hydrogen atoms even when the total hydrogen content of the cluster decreases. In particular, Fe is found to migrate from the surface sites to the interior sites during the dehydrogenation process, releasing more hydrogen as it diffuses. This diffusion mechanism may account for the fact that a small amount of catalysts is sufficient to improve the kinetics of MgH2, which is essential for the use of this material for hydrogen storage in fuel-cell applications

  • 76.
    Larsson, Peter O.
    et al.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Larsson, Andreas
    Aruja, Rajeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Ding, Feng
    Physics Department, Göteborg University.
    Yakobson, Boris I.
    ME and MS Department, Rice University, Houston.
    Duan, HaiMing
    Physics Department, Göteborg University.
    Rosén, Arne E.
    Physics Department, Göteborg University.
    Bolton, Kim
    Physics Department, Göteborg University.
    Calculating carbon nanotube-catalyst adhesion strengths2007Ingår i: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, nr 11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Density-functional theory is used to assess the validity of modeling metal clusters as single atoms or rings of atoms when determining adhesion strengths between clusters and single-walled carbon nanotubes (SWNTs). Representing a cluster by a single atom or ring gives the correct trends in SWNT-cluster adhesion strengths (Fe≈Co>Ni), but the single-atom model yields incorrect minimum-energy structures for all three metals. We have found that this is because of directional bonding between the SWNT end and the metal cluster, which is captured in the ring model but not by the single atom. Hence, pairwise potential models that do not describe directional bonding correctly, and which are commonly used to study these systems, are expected to give incorrect minimum-energy structures

  • 77.
    Li, Yunguo
    et al.
    Applied Material Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH).
    Ahuja, Rajeev B.
    Applied Material Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH).
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Communication: Origin of the difference between carbon nanotube armchair and zigzag ends2014Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 140, nr 9, artikel-id 91102Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this work, we have found that the difference between armchair and zigzag ends of carbon nanotubes (CNTs) does not pertain at close study for individual bonds and thus alternative strategies need to be developed to reach the ultimate goals in selective growth. Based on first-principles simulations, the difference between binding strengths for CNTs of different chirality was investigated using hydrogen dissociation energies at their passivated ends. When all H atoms are removed collectively we find the well-known difference: that armchair bonds are much weaker than zigzag ones, which is typically seen for both CNT ends and graphene edges. However, when individual H atoms are removed we find almost no difference in hydrogen dissociation energies, small difference in bond lengths, which by association means small difference in C-C and M-C binding energies. We show convincingly that the difference in binding energy between armchair and zigzag ends is due to a fragment stabilization effect that is only manifested when all (or several neighbouring) bonds are broken. This is because at armchair ends/edges neighbouring dangling bonds can pair-up to form C≡C triple bonds that constitute a considerable stabilization effect compared to the isolated dangling bonds at zigzag ends/edges. Consequently, in many processes, e.g., catalytic growth where bonds are normally created/broken sequentially, not collectively, the difference between armchair and zigzag ends/edges cannot be used to discriminate growth of one type over the other to achieve chiral selective growth. Strategies are discussed to realize chirality selective growth in the light of the results presented, including addition of C2-fragments to favor armchair tubes

  • 78.
    Li, Zhong
    et al.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    Larsson, Andreas
    Larsson, Peter O.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Ahuja, Rajeev B.
    Department of Physics, Condensed Matter Theory Group, Uppsala University.
    Tobin, Joseph M.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    O'Byrne, Justin P.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    Morris, Michael A.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    Attard, Gary Anthony
    School of Chemistry, Cardiff University.
    Holmes, Justin D.
    Department of Chemistry, Materials Section and Supercritical Fluid Centre, University College Cork.
    Copper/molybdenum nanocomposite particles as catalysts for the growth of bamboo-structured carbon nanotubes2008Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 112, nr 32, s. 12201-12206Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Bamboo-structured carbon nanotubes (BCNTs), with mean diameters of 20 nm, have been synthesized on MgO-supported Cu and Mo catalysts by the catalytic chemical vapor deposition of methane. BCNTs could only be generated using a combination of Cu and Mo catalysts. No BCNTs were produced from either individual Cu or Mo catalysts. In combination, Mo was found to be essential for cracking the methane precursor, while Cu was required for BCNT formation. Energy dispersive X-ray analysis of the individual particles at the tips of the nanotubes suggest that Cu and Mo are present as a "composite" nanoparticle catalyst after growth. First-principles modeling has been used to describe the interaction of the Cu/Mo catalyst with the nanotubes, suggesting that the catalyst binds with the same energy as traditional catalysts such as Fe, Ni, and Co

  • 79.
    Löfgren, Robin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Pawar, Ravinder
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Charged dopants in neutral supercells through substitutional donor (acceptor): nitrogen donor charging of the nitrogen-vacancy center in diamond2018Ingår i: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 20, artikel-id 023002Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Charged defects are traditionally computed by adding (subtracting) electrons for negative (positive) impurities. When using periodic boundary conditions this results in artificially charged supercells that also require a compensating background charge of the opposite sign, which makes slab supercells problematic because of an arbitrary dependence on the vacuum thickness. In this work, we test the method of using neutral supercells through the use of a substitutional electron donor (acceptor) to describe charged systems. We use density functional theory (DFT) to compare the effects of charging the well-studied NV-center in diamond by a substitutional donor nitrogen. We investigate the influence of the donor-N on the NV-center properties as a function of the distance between them, and find that they converge toward those obtained when adding an electron. We analyze the spin density and conclude that the donor-N has a zero magnetic moment, and thus, will not be seen in electron spin resonance. We validate our DFT energies through comparison to GW simulations. Charging the NV-center with a substitutional donor-N enables accurate calculations of slabs, without the ambiguity of using charged supercells. Implantation of donor-N atoms opens up the possibility to engineer NV-centers with the desired charge state for future ICT and sensor applications.

  • 80.
    Löfgren, Robin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Pawar, Ravinder
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Density functional theory calculations on the diamond nitrogen-vacancy center2015Konferensbidrag (Övrigt vetenskapligt)
  • 81.
    Löfgren, Robin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Pawar, Ravinder
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Larsson, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    The bulk conversion depth of the NV-center in diamond: computing a charged defect in a neutral slab2019Ingår i: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 21, artikel-id 053037Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The negatively charged nitrogen vacancy (NV-) center in diamond has properties that make it a promising candidate for applications such as a qubit in room temperature quantum computing, single-molecule photoluminescence and NMR sensor, and as a single photon source for quantum cryptography. For many of its uses it is desirable to have the NV-center close to the diamond surface. In this work, we use density functional theory simulations to investigate how the distance of the NV- center to a surface, and its orientation, affect its properties, including the zero-phonon-line. We study the three technologically important surfaces terminated with fluorine, oxygen/hydroxyl and nitrogen. Since the NV-center is charged it requires special measures to simulate within a slab-model. We use the recently proposed charging with a substitutional donor in the diamond lattice resulting in a neutral super-cell, which provides very satisfactory results. We have found that the NV-centers properties converge to bulk values already at 5 angstrom depth.

  • 82.
    Melchor, Santiago
    et al.
    Departamento Quimica Organica, Campus Fuentenueva, Universidad de Granada.
    Dobado, J.A.
    Departamento Quimica Organica, Campus Fuentenueva, Universidad de Granada.
    Larsson, Andreas
    Greer, James C.
    NMRC, University College, Lee Maltings, Cork.
    Bonding of atomic phosphorus to polycyclic hydrocarbons and curved graphitic surfaces2003Ingår i: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 125, nr 8, s. 2301-2306Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a theoretical study of the bonding of atomic phosphorus to planar hydrocarbons and to curved graphite-like surfaces. We find that bonding of phosphorus to planar polycyclic hydrocarbons induces curvature away from the phosphorus atom, as defined by the pyramidalization angle. Similarly, bonding of atomic phosphorus to the [5,5] fulvalene-circulene semifullerene and buckminsterfullerene is only possible on the convex side of the carbon surface. On the other hand, we find the interaction of atomic phosphorus with the concave side of fullerene-like surfaces to be nonbonding for both quartet and doublet spin states. We find the prerequisite for stable epoxy-type bonds within these systems is the ability of the carbon atoms to maintain or induce curvature away from the P···C=C bond.

  • 83.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Density Functional Characterization of Ce2@C80: Explanation of the Ce Preferential bonding site inside the Ih-C80 cage2008Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    Metallofullerenes have many possible uses in technology and even in bio-medical applications.1 Some fullerenes have been found to have two incarcerated metal atoms such as M2@C80 (M = La; Ce; etc.).2 Owing to the difficulty in synthesis and purification; structural characterisation has not been reported for many fullerenes that has two metal atoms. Here in this study we report the structural and electronic properties of Ce2@C80; calculated using density functional theory. Ce in Ce2@C80 is found to have a novel binding site which minimizes Ce(f) Ce(f) overlap to favor Ce-C bonding leading to D3d configuration while La in La2@C80 has D2h symmetric ground state structure.3 (Picture) HOMO and LUMO of D3d Ce2@C80 The analysis of the binding energy of Ce shows that the two endohedral atoms have a weaker binding in Ih-C80 than it has in Ce@C80 and in Ce@C82.4 We discuss the reason for this preference of binding site where Ce has a weaker binding strength; using the frontier level orbitals of Ce2@C80 in comparison with the orbitals of La2@C80. Further; the nature of charge transfer and the oxidation state of the Ce atoms in Ce2@C80; correlating to the conventional Ce tri-halide compounds (CeF3;CeCl3) will be explained. The investigation on the simulated vibrational spectrum will also be presented

  • 84.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Density Functional Theory Description of the Preferential Binding Site of Cerium Atoms in Ce2@C802008Konferensbidrag (Refereegranskat)
    Abstract [en]

    iCeC80; Ce2@C80; Cerium; DFT; Electronic Structure Theory; Turbomole Abstract: Fullerenes encapsulating one or more lanthanoides such as M@C82 or M2@C80 (M = La; Ce; etc.) have interesting properties for potential applications as devices in nanoelectronics and contrasting agents in biomedical sciences.1-3 The properties of these species; like electronic structure; stability and the symmetry of the molecule; depends upon the position of the metal atom inside the cage. In this study we characterize Ce2@C80 through density functional theory calculations and establish the competeting nature of binding between the two Ce atoms and Ce-C bonding; the nature of which causes cerium bond to a different binding site in Ih-C80 compared to fullerenes with only one incarcerated Ce atoms. We observe a strong hybridization between the d orbitals of cerium and the π electrons of the Ih-C80 cage. Here we report and discuss the ground state configurations of Ce2@C80 in connection with our recent study of Ce@C82.4;5We; in addition; show that Ce2@C80 has a different ground state geometry than La2@C80.6

  • 85.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Density Functional Theory Explanation of the Different Preferential Binding Sites for Ce and La In M2@C80 (M=Ce; La)2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    Fullerenes encapsulating metal atoms have many possible uses in technology and even in bio-medical applications.1 Some fullerenes have been found to have two incarcerated metal atoms such as M2@C80 (M = La; Ce; etc.).2 Here we report the structural and electronic properties of Ce2@C80 calculated using density functional theory. We have found that Ce2@C80 has a D3d symmetric ground state structure and that Ce binds to a different type of binding site compared to other cerium containing fullerenes; such as Ce@C823 and even in Ce@C80. Fig: This binding site also differs compared to La in La2@C80; which is D2h symmetric. Since each of the six-membered rings in Ih-C80 satisfies the proposed criteria for (M=La; Ce) binding3; we here analyze and discuss the reason of Ce binding to another site based on its frontier level orbitals in connection with the orbitals of La2@C80. We also discuss oxidation state of Ce in Ce2@C80; in comparison with the traditional Ce tri-halides compounds (CeCl3; CeF3) and the investigation on the simulated vibrational spectrum of the ground state isomer of Ce2@C80.

  • 86.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Density Functional Theory Prediction of the Different Binding Sites For Ce in C78; C80 and C82 Cages Insights Through Electronic Structure2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    Fullerenes; that have one metal atom encapsulated; are relatively well studied compared to species that have two elements inside the cage.1-5 Cerium; the most reactive elements of the rare earth group can be encapsulated into fullerene (C78; C80; C82) cages;1 which works as a n-dopant and in the incarcerated form it can be used as quantum bits in quantum computing2. Understanding of the position and the nature of binding of the metal atom inside the cage is important as it controls the structural and electronic properties of the molecule which in turn is essential in suggesting a novel candidate in the field of molecular electronics. Based on our DFT calculations; we have found that Ce prefers a specific and unique binding site in C82-C2v and has a C2v symmetric structure which we explain by the specific charge pattern of this binding site and the symmetry of the MO s that comply well with the Ce d orbital bonding.6 Analysis of all six membered rings of C80-Ih shows that all these rings have the distinct charge pattern; and therefore it is anticipated that two Ce atoms in Ce2@C80 should adapt to D2h symmetry where two Ce atoms binds with a center of a six-membered ring with a maximum distance due to the nature electrostatic repulsion between these two Ce atoms. But our DFT calculations show that Ce atoms in Ce2@C80 prefer a D3d configuration where two Ce atoms reside on-top of a carbon atom on the C3 axis of C80-Ih. We explain this fact by analyzing the electronic structure and expect similar things to happen in Ce2@C78. But in Ce2@C78 unlike Ce2@C80; Ce has its binding pattern as in Ce@C82 (binding to a center of a six-membered ring). We explain this variation in binding by analyzing its electronic structure and also explain the nature of the charge transfer between the Ce atoms and the cage (C82 and C80).

  • 87.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Endohedral fullerenes and their applications in technology2009Ingår i: Advances in nanotechnology and applications, Louisville, KY: Center for Nanotechnology, Research, & Applications (CENTERA), Sullivan University College of Pharmacy , 2009, s. 127-135Kapitel i bok, del av antologi (Refereegranskat)
  • 88.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Schulte, Karina H.G.
    MAX-lab, Lund University.
    Moriarty, Philip J.
    School of Physics and Astronomy, University of Nottingham.
    Stróżecka, Anna
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Voigtländer, Bert
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Cerium Endohedral fullerenes (Ce@C82 and Ce2@C80) Theoretical Interpretations for Experimental observation2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    Many questions remain unanswered for the endohedral fullerenes. Owing to the improvement in separation of isomers and in theoretical studies (DFT and ab-initio) evolving as imperative tool for characterization; these can be addressed.1;2 Understanding of the position and binding configuration of the metal atom inside the cage is crucial as it controls the structural and electronic properties of the molecule.3 Theoretical calculations proved to be efficient in explaining many controversies in the field of lanthanoid endohedral fullerenes.2-5 Fig; Ce2@C80; Ce2@C78) DFT optimized structures of Ce2@C80 D3d and Ce2@C78 D3h Here in this study we use DFT to characterize Ce doped metallofullerenes and report some surprising theoretical findings on the binding of cerium inside various carbon cages; (C60; C78; C80; C82). We observe that the presence of an additional Ce atom puts restrictions on the binding in the C80 cage6; but this does not happen in the C78 cage. We explain the reason behind this by analyzing the electronic structure. Further various spectra (RESPES; IETS; STM/STS) have been simulated for Ce@C82 and Ce2@C80 which we compare and discuss with experiments

  • 89.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Schulte, Karina H.G
    Moriarty, Philip J.
    Stróecka, Anna
    Institut für Bio- und Nanosysteme, Forschungszentrum Jülich.
    Voigtländer, Bert
    Institut für Bio- und Nanosysteme, Forschungszentrum Jülich.
    Surprising Findings for Ce Doped Fullerenes and Understanding of Experiments through Theoretical Modelling A DFT Approach2008Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    Endohedral fullerenes; which have one metal atom encapsulated; are relatively well studied compared to species that have two elements inside the cage.1-5 Cerium; the most reactive elements of the rare earth group can be encapsulated into fullerene (C78; C80; C82) cages;1 which works as a n-dopant and in the incarcerated form it can be used as quantum bits in quantum computing2. Understanding of the position and binding pattern of the metal atom inside the cage is important as it controls the structural and electronic properties of the molecule. We found that Ce in Ce@C82 has a specific and unique binding site in C82-C2v and has a C2v symmetric structure; which is explained by the specific charge pattern of this binding site and the symmetry of the MO s that comply well with the Ce d orbital bonding.6 This six-membered ring binding site is also favored by La in La@C82. Each of the six-membered rings of C80-Ih fulfills this symmetry criterion and therefore a similar kind of binding site is expected for Ce in Ce2@C80. But; we observe a novel binding site for Ce in presence of an additional cerium atom; while La preserve its usual binding pattern in La2@C80 as in La@C82.7 We here discuss and analyze the reason for the preference of novel binding site of Ce atoms in C80-Ih by explaining the competitive binding nature of Ce-Ce and Ce-C. Surprisingly; Ce in Ce2@C78 unlike Ce2@C80 has its binding pattern as in Ce@C60 and in Ce@C82 (binding to a six-membered ring). We explain this variation in binding together with the nature of the charge transfer between the Ce atoms and the cage (C82 and C80). In addition; we explain experimental observations for Ce@C82 and Ce2@C80 from RESPES; IETS; STM/STS spectra by comparison with simulated properties

  • 90.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, Computational Modelling Group, University College Cork.
    Larsson, Andreas
    Strózecka, Anna
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Voigtländer, Bert
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Cerium doped Fullerenes (Ce@C82 & Ce2@C80) adsorbed on Cu(111) surface: Theoretical Explanation for Surprising Experimental Findings2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    Cerium is one of the most reactive elements of the rare earth group; and the complex physics accompanying its single occupied 4f band; close to the Fermi level; has baffled scientists for many decades. Cerium can be encapsulated into fullerene (C82 & C80) cages;1-4 which works as an n-dopant and in the incarcerated form it can be used as quantum bits in quantum computing2. Theoretical studies aiming at the detailed picture of metal-cage interaction and a closer look on the electronic structure of these Ce doped fullerenes have been reported.3-4 But; only a few theoretical studies have been conducted with an aim to address the nature of interaction of these endohedral fullerene molecules with semiconductors and metal surfaces. Hence; in this study; we apply density functional theory (DFT) to expand the investigation of Ce@C82 and Ce2@C80 on Cu (111) surface to have a more detailed study from both a fundamental and applied viewpoint to what extent the fullerene-surface interaction influences the encapsulated atom. Our calculations reveal a surprising observation in the electronic structure of doped Ce fullerenes on Cu (111) surface. We discuss and elucidate our results with the measured STM/STS spectra

  • 91.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Stróżecka, Anna
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Viogtländer, Bert
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    A Joint Theoretical and Experimental Characterization of the In-elastic Tunneling modes of Ce@C82 and Ce2@C802008Konferensbidrag (Refereegranskat)
    Abstract [en]

    The drive for miniaturization of electronic devices involves nanotechnology; where single molecules are investigated for the use as electric components.1 Owing to their electronic and magnetic flexibility; endohedral fullerenes have been suggested as one of a possible molecular candidate in future electronics.2 Such molecular components differ from the conventional electronic materials by the degree to which mechanical degrees of freedom affect electrical conductivity.3;4 Understanding of these processes are still in its infancy and in this study in-elastic tunnelling spectroscopy (IETS) which probes the tunnelling current of a single molecule using scanning tunnelling microscopy (STM); with controlled excitation of their vibration modes has been applied to cerium endohedral fullerenes (Ce@C82 and Ce2@C80) to investigate the interplay between electrons and vibrations of these molecules on Cu(111).5 We report here our measured IETS spectrum of single Ce@C82 and Ce2@C80 on Cu (111) and explain the observed IETS peaks by analysing the molecular vibrational spectra simulated using density functional theory calculations.

  • 92.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Stróżecka, Anna
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Voigtländer, Bert
    Institut für Bio- and Nanosysteme (IBN 3) and CNI – Forschungszentrum Jülich.
    Schulte, Kristina H.G.
    AX-lab, Lund University.
    Moriarty, Philip J.
    School of Physics and Astronomy, University of Nottingham.
    Theoretical Predictions and Explanation of Experimental Observations for Ce Doped Fullerenes2008Konferensbidrag (Refereegranskat)
    Abstract [en]

    Fullerenes that have one metal atom encapsulated; for example M@C60; M@C70; M@C82; (M= Sc; Y; La; Ce; etc.) are relatively well studied compared to species that have two elements inside the cage.1-4 We recently reported the structure of Ce@C82 and explained the preferential binding site of Ce to only one of the thirty-one 6-membered rings of C82-C2v cage by identifying its specific charge pattern and the symmetry of the MO s that comply well with the Ce d orbital bonding.5 Since; each of the six-membered rings of C80-Ih fulfill the proposed criteria; similar kind of binding site is expected for Ce in Ce2@C80. But; we observe a novel binding site for Ce in presence of an additional cerium atom; while La in La2@C80 does bond with six-membered rings.6 In this study; we discuss and analyze the reason for the preference for a novel binding site by Ce atoms in C80-Ih through density functional calculations. Further; we explain the nature of the charge transfer between the Ce atoms and the cage (C82 and C80) and elucidate the oxidation state of Ce in these metallofullerenes by comparing the charge transfer in the conventional Ce tri halides (CeF3; CeBr3). In addition; we explain experimental observations for Ce@C82 and Ce2@C80 from RESPES; IETS; STM/STS spectra by comparison with simulated properties

  • 93.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Stróecka, Anna
    Institut für Bio- und Nanosysteme, Forschungszentrum Jülich.
    Voigtländer, Bert
    Institut für Bio- und Nanosysteme, Forschungszentrum Jülich.
    Larsson, Andreas
    Endohedral fullerenes and their implications in technology2009Konferensbidrag (Refereegranskat)
    Abstract [en]

    The interest in nanoscale materials is due to their positive impact in various scientific areas such as energy; environmental and in biomedical sciences. Carbon has a wide range of interesting properties; which makes it a versatile element and special attention has been paid to carbon based nanostructures such as fullerenes and nanotubes among several other investigated materials. Fullerenes; with their distinctive carbon cage structure; have been the object of intense research since their discovery and they have become a building block of nanoscience and a base for novel molecular materials. Fullerenes have also played a central role in the development of a variety of single molecule techniques and spectroscopies; each of which is currently an intensely active sub-field of nanoscience and nanotechnology. These include: single molecule manipulation; single molecule electronic and optical microscopy; and monomolecular electronic devices

  • 94.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Stróżecka, Anna
    Peter Grünberg Institut (PGI-3), and JARA-Fundamentals of Future Information Technology, Forschungszentrum Jülich.
    Mysliveček, Josef
    Charles University, Faculty of Mathematics and Physic.
    Dybek, Aneta
    Department of Physics, Queen Mary University of London.
    Dennis, John S.
    Department of Physics, Queen Mary University of London.
    Voigtländer, Bert
    Peter Grünberg Institut (PGI-3), and JARA-Fundamentals of Future Information Technology, Forschungszentrum Jülich.
    Larsson, Andreas
    Endohedral Fullerene Ce@C82 on Cu(111: Orientation, Electronic Structure, and Electron-Vibration Coupling2013Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, nr 4, s. 1656-1662Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Structural, electronic, and vibrational properties of the endohedral fullerene Ce@C82 on Cu(111) have been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). Ce@C82 forms islands on the substrate. Our STM measurements show relatively large differences in morphology and electron spectra of molecules within these islands indicating multiple molecular orientations on the substrate, while the vibrational spectra are more uniform. We have determined molecular orientations by comparing STM and DFT molecular morphology, and we have calculated Ce@C82 bound to Cu(111) and found that it is chemisorbed. We show that Ce@C82 adopts orientations on the surface that enables Ce to remain at its most favorable binding site inside C82. The effect of chemisorption on the structural and electronic properties of Ce@C82 is thus small, and the orientations are limited to configurations with Ce in the upper hemisphere of the molecular configurational space. We show that the variations in the dI/dV spectra between molecules of different orientations is due to Ce-cage orbitals that are localized in space and their involvement in tunneling depends on the molecular orientation on the substrate. The observed electron-vibration coupling modes in the STM-IETS (in-elastic tunneling spectroscopy) of Ce@C82 arise from cage modes only, and therefore, electron transport properties are expected to be different compared to Ce2@C80, which has active Ce-cage vibrations.

  • 95.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    A density functional study of Ce@C82: Explanation of the Ce preferential bonding site2008Ingår i: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 112, nr 5, s. 1071-1075Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ce has been found experimentally to be preferentially incorporated into the C82 isomer of C2v symmetry as have other lanthanoids in M@C82 (M = La, Pr, Nd, etc.). We have investigated the underlying reason for this preference by calculating structural and electronic properties of Ce@C82 using density functional theory. The ground-state structure of Ce@C82 is found to have the cerium atom attached to the six-membered ring on the C2 axis of the C82-C2v cage, and the encapsulated atom is found to perturb the carbon cage due to chemical bonding. We have found Ce to favor this C2v chemisorption site in Cv2 by 0.62 eV compared to other positions on the inside wall of the cage. The specific preference of the metal atom to this six-membered ring is explained through electronic structure analysis, which reveals strong hybridization between the d orbitals of cerium and the π orbitals of the cage that is particularly favorable for this chemisorption site. We propose that this symmetry dictated interaction between the cage and the lanthanide d orbital plays a crucial role when C82 forms in the presence of Ce to produce Ce@C82 and is also more generally applicable for the formation of other lanthanoid M@C82 molecules. Our theoretical computations are the first to explain this well-established fact. Last, the vibrational spectrum of Ce@C82 has been simulated and analyzed to gain insight into the metal-cage vibrations

  • 96.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    DFT Study of Ce2@C80 Comparison with La2@C802007Konferensbidrag (Refereegranskat)
    Abstract [en]

    Many rare earth elements have been encapsulated in fullerene cages and their possible applications include quantum bits in quantum computing; MRI agents in bio-medical sciences and as superconductors; which have made them interesting species both as a molecule and as a material.1 Among such available fullerenes; Ih-C80 has interesting chemistry as the unstable isomer gets stabilized by encapsulating two metal atoms such as M2@C80 (M=La; Ce etc.).3 Ce2@C80 is one such species and is reported to be extracted over a decade ago; but until now only a few studies exploring its nature have been reported.4;5 Top and Side view of Ce2@C80 Hence; we have applied density functional theory to expand the investigation of Ce2@C80 further to have a closer look at its structural and electronic properties. Our calculations reveal preferential binding sites of the two Ce inside the Ih-C80 cage; which differs from other cerium containing fullerenes such as Ce@C82.6 The reason for this will be discussed and compared with experiments and with the results available for La2@C80.

  • 97.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    DFT Study of Ce2@C82: comparison betwwn cerium and lanthanum inside C822006Konferensbidrag (Refereegranskat)
  • 98.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Electronic Structure and Vibrational Spectrum of Ce@C82 – Insights through DFT Computations2006Konferensbidrag (Refereegranskat)
  • 99. Muthukumar, Kaliappan
    et al.
    Larsson, Andreas
    Endohedral Metallofullerenes: Novel Materials for Nanoelectronics: Insights from Density Functional Theory Modeling2010Bok (Refereegranskat)
    Abstract [en]

    Fullerenes, hollow carbon cages on the order of 1 nm in diameter, are future building blocks in many areas of applied and fundamental nanoscience. Fullerenes show potential for applications in nanotechnology due to the possibility of tuning their properties by doping and/or functionlization. In particular, the endohedral doping of the hollow carbon cage with metal atoms and clusters allows changing of the electronic and magnetic properties of the molecule without significant distortion of the geometry of the outer shell. Despite their outstanding applications in technology and in bio-medical sciences the limited availability of endohedral metallofullerenes has hindered studies that describe their electronic and structural properties, the knowledge of which are essential prerequisites for many nanotechnological applications. In this book, we present our results on the characterization (structural, electronic and vibrational) of cerium endohedral fullerenes and provide precious clues about the formation mechanism of these species at the nanoscale.

  • 100.
    Muthukumar, Kaliappan
    et al.
    Tyndall National Institute, University College Cork.
    Larsson, Andreas
    Explanation of the different preferential binding sites for Ce and la in M2@C80 (M = Ce, La2008Ingår i: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 18, nr 28, s. 3347-3351Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Metallofullerenes have many possible uses in technology and even in bio-medical applications. Some fullerenes have been found to have two incarcerated metal atoms such as M2@C80 (M = La, Ce, etc.). We have calculated the structural and electronic properties of Ce 2@C80 using density functional theory (DFT). Ce is known to be preferentially incorporated into the Ih symmetric C 80 isomer as La does in La2@C80. We have found that Ce2@C80 has a D3d symmetric ground state structure and that Ce binds to a different type of binding site compared to other cerium containing fullerenes, such as Ce@C82. This binding site also differs compared to La in La2@C80, which is D 2h symmetric. The two Ce atoms inside the C80-I h cage are equivalent and retain their f-electron. The Ce atoms bind on-top of one carbon atom (and its three neighbors) in Ce2@C 80, compared to in the center of a six-membered ring as in C 82. This novel binding site minimizes Ce(f)⋯Ce(f) overlap in favour of Ce-C bonding, giving the D3d configuration

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